Since silicone rubber compositions that contain a calcium carbonate powder and are cured by a hydrosilylation reaction also contain an alkali component as an impurity of said calcium carbonate powder, a problem arises during storage of such compositions because they generate gaseous hydrogen as a result of a reaction with an organopolysiloxane having silicon-bonded hydrogen atoms that is included in the composition as a curing agent. As a solution to this problem, Patent Document 1 proposes a silicone rubber composition that comprises a diorganopolysiloxane having at least two alkenyl groups in one molecule, an organopolysiloxane having at least two silicon-bonded hydrogen atoms in one molecule, a platinum metal-type catalyst, and a calcium carbonate powder surface-treated with a partially hydrolyzed condensate of tetraalkoxysilane. As other solutions to this problem, Patent Documents 2 and 3 propose silicone rubber compositions that comprise a diorganopolysiloxane having at least two alkenyl groups in one molecule, a calcium carbonate powder substantially surface-treated with diorganopolysiloxane, an organopolysiloxane having at least two silicon-bonded hydrogen atoms in one molecule, and a platinum metal-type catalyst.
In these cases, it is typical to divide addition reaction curing silicone rubber compositions into two parts for storage (Patent Documents 3 and 4 (Japanese Unexamined Patent Application Publication No. 2006-117823)) and dividing calcium carbonate-containing addition reaction curing silicone rubber compositions into two parts is also proposed. Furthermore, Patent Document 1 described above proposes division of the composition into a first liquid comprising the alkenyl group-containing diorganopolysiloxane, the platinum metal-type catalyst, and the calcium carbonate surface-treated with a partially hydrolyzed condensate of tetraalkoxysilane and a second liquid comprising the alkenyl group-containing diorganopolysiloxane and an organohydrogenpolysiloxane. Moreover, Patent Document 2 proposes division of the composition into a first liquid comprising the alkenyl group-containing diorganopolysiloxane and the calcium carbonate surface-treated with the diorganopolysiloxane and a second liquid comprising an organohydrogenpolysiloxane, the platinum metal-type catalyst, and an adhesion-imparting agent.
Patent Documents 5 and 6 propose providing a two-part silicone rubber composition in which calcium carbonate is not mixed with a liquid comprising an organohydrogenpolysiloxane having, on average, at least two SiH groups in one molecule whereby, even when storing the two liquids for an extended period of time, changes in viscosity of a composition comprising a curing agent are small, the liquids can be uniformly mixed at a volume ratio of 1:1 using a motionless mixer such as a static mixer or the like, and physical properties of the silicone rubber and adhesion to silicone rubber can be obtained as initially planned.
However, there is no recitation in any of these patents regarding changes in physical properties and/or adhesion in cases where the volume ratio, when forming a silicone rubber forming composition by mixing the separately stored first composition and second composition, varies.